1. Field of the Invention
The present invention generally relates to the field of bistatic radar cross-section (RCS) measurement apparatus and, more particularly, to a bistatic RCS measurement system capable of reducing contaminating bistatic clutter.
2. Description of the Related Art
Bistatic radars and consequently bistatic RCS measurements have existed from the advent of radar nearly 80 years ago. In a typical measurement, a transmit antenna is used to direct a beam of electromagnetic radiation at a target supported by a low-scattering structure and then a receive antenna measures the scattered radiation to determine the RCS. The antennas are generally mounted on towers to elevate them above the ground and thereby minimize the clutter introduced by the ground. It is noted the radiation reflected off the ground is sometimes used to provide a signal strength enhancement on large ranges, but care is taken to create a flat, clutter-free reflecting surface. In either case the bistatic receive antenna is placed at a different location than the transmit antenna to sample the radiation scattered from the target at a bistatic angle measured in the plane defined by the transmit antenna, target, and receive antenna.
Bistatic RCS measurements may be split into two categories. Either the bistatic antenna is held at a fixed location with respect to the transmit antenna and target while the target is rotated or the target is held at a fixed pose while the bistatic antenna is rotated around the target. The former is known as a “fixed-angle bistatic measurement” and the later as a “variable-angle bistatic measurement.” In both situations the mechanism supporting the bistatic antenna and receiver can provide a significant source of clutter that is difficult to remove with RF gating because of the range geometry and signal path timing. Therefore, the antenna support structures are usually covered with radar absorbing material (RAM) to minimize the resulting clutter. For many targets, the attenuation provided by the RAM material is insufficient to reduce the structural scattering to levels that do not significantly contaminate the desired signal. Also, the complex measurement range geometries encountered in bistatic measurements make shaping the support structures so as to redirect the reflected radiation away from the receive antenna structure for all angles virtually impossible.
Accordingly, there is a need in the art for an improved bistatic measurement apparatus.